Climatic control of mat vegetation communities on inselberg archipelagos in south-eastern Brazil

Granite and/or gneiss inselbergs are excellent examples of geomorphologically stable island habitats, considered as old, climatically buffered, infertile landscapes (OCBILs). However, unlike oceanic islands, their underlying drivers of diversity patterns remain to be investigated. Here, we studied 24 inselbergs in south-eastern Brazil, aiming to understand the role of landscape variables and environmental conditions in the assembly of the characteristic extremophilic mat vegetation communities. We found that beta diversity was largely explained by climatic variables, whereas species richness did not vary among inselbergs. Classic determinants of the diversity of island communities do not generally seem to apply to these plant assemblages. Overall, these communities change along a coast-to-inland gradient that captures increased seasonality with a replacement of more hydrophilic taxa by more drought-tolerant taxa. Changes in species composition in space involved strong species replacement, with several widespread genera locally represented on distinct inselbergs by different narrowly distributed species. Despite the deterministic sorting of taxa based on climatic conditions, a substantial fraction of the beta diversity remained unexplained. This underlines the importance of historical processes, which are easier to notice in stable OCBIL regions, such as range expansion, local extinction, dispersal constraints and allopatric speciation.